1. Field of the Invention
The present invention concerns a method of assembling an optical module. An optical module is defined as an element comprising at least two optical components each including a waveguide, the two optical components being optically coupled to each other. A plurality of optical fibers can be connected to an optical module of the above kind.
2. Description of the Prior Art
Optical modules have already been produced in the prior art. One of the main problems to be solved in assembling such modules is aligning the waveguides of the various optical components to achieve high quality optical coupling.
The waveguides of optical components have very small dimensions. For example, a laser component 100 .mu.m thick, 500 .mu.m long and 250 .mu.m wide has a waveguide buried in its structure that is in the order of 0.2 .mu.m thick, 2 .mu.m wide and 500 .mu.m long, for example. A guide of the above kind must be aligned with another waveguide whose section also has small dimensions, for example in the order of 10 .mu.m.sup.2 or even less. The problem is therefore to align such waveguides having small dimensions, especially if they are surrounded by relatively bulky optical components.
The optical modules are generally produced either on a silicon (Si) support structure or on a silica-on-silicon (SiO.sub.2 /Si) support structure. FIGS. 1 to 3B are diagrammatic representations of various embodiments of conventional optical modules formed on an SiO.sub.2 /Si support structure. FIGS. 4 to 6B are diagrammatic representations of various embodiments of conventional optical modules formed on an Si support structure.
FIG. 1 is a diagrammatic exploded perspective view of a conventional optical module and FIGS. 2A and 2B are diagrammatic representations of the front face of two optical components respectively separate and then assembled in the optical module from FIG. 1. The module is formed on an Si0.sub.2 /Si support structure including a silicon substrate 1 onto which are stacked two silica layers 2, 3. The second silica layer 3 is etched to a particular shape, for example in the form of a beam, and constitutes an optical waveguide of a first optical component 30 integrated into the structure.
A trench 5 is defined in the silica layers 2, 3 and near the substrate 1 to accommodate a second optical component 6 including a waveguide 7. The second optical component can be a laser or an amplifier, for example, or a photodetector or any other active or passive component. It is placed in the trench 5 and fixed to the substrate 1, in the bottom of the trench, by means of a spot of solder S. It is disposed so that the optical axis A7 of its waveguide 7 is aligned with the optical axis A3 of the waveguide 3 of the first optical component 30.
Light is emitted perpendicularly to the front face of the module, along the axis designated x in the figures. The second optical component 6 is disposed in the trench 5 so that the optical axes of the waveguides 3 and 7 are vertically and horizontally aligned with the axes designated z and y, respectively.
A routine method of aligning the waveguides 3 and 7 is to control the height h of the spot of solder S. This is very difficult. Consequently, this method provides only approximate control of the height of the waveguide 7 of the second optical component 6 relative to the waveguide 3 of the first optical component 30. Also, this method cannot easily control transverse alignment of the waveguides, i.e. alignment along the y axis. Transverse alignment must therefore be carried out with the naked eye or optically (the alignment process is slow given the required level of accuracy) or by reflow of the solder (poor accuracy, in the order of 3 .mu.m).
This assembly method relying on controlling the height h1 of the spot of solder S1 between the two optical components has also been used in modules produced on Si structures and as shown in FIGS. 4, 5A and 5B. FIG. 4 is a diagrammatic exploded perspective view of a module of this kind and FIGS. 5A and 5B are diagrammatic representations of the front face of the two optical components, respectively separate and then assembled in the module from FIG. 4. The module is produced on a silicon substrate 10 in which a V-shaped groove 11 is chemically etched to house an optical fiber 12. The optical fiber 12 includes a core 13 (optical waveguide) and forms the first optical component of the module. A trench 15 is formed in the Si substrate 10 to house a second optical component 16 including a waveguide 17.
Another solution has been proposed to improve the quality of the alignment of the waveguides and in particular to allow transverse alignment, along the y axis. This solution is shown diagrammatically in FIGS. 3A, 3B and 5A, 5B, which respectively correspond to the optical module formed on an SiO.sub.2 /Si support structure and to the optical module formed on an Si structure.
The solution consists in providing a first contact surface 8 (18) in the trench in the SiO.sub.2 /Si (Si) structure and a second contact surface 9 (19) on the second optical component 6 (16), the second contact surface 9 (19) being adopted to cooperate with the first contact surface 8 (18). In this case, alignment is no longer defined by the height of the spot of solder, which can therefore be less precise. The contact surfaces respectively define vertical bearing planes 8a (18a), 9a (19a) along the z axis and lateral planes 8b (18b), 9b (19b) along the y axis and it is the position of these bearing planes that controls the position of the waveguides relative to each other. The contact surface 8 is lithographically etched into the first silica layer 2 of the SiO.sub.2 /Si support structure after etching the guide 3 in the form of a beam.
As shown in FIGS. 3A, 3B and 6A, 6B, the second optical component 6 (16) is placed in the trench in the SiO.sub.2 /Si (Si) support structure so that its contact surface 9 (19) is pressed against the contact surface 8 (18) of the SiO.sub.2 /Si (Si) support structure. The position of the contact surface 8 (18) of the SiO.sub.2 /Si (Si) support structure is adjusted to align the waveguides 3 (13) and 7 (17) transversely, along the y axis, and vertically, along the z axis.
The surface of the SiO.sub.2 /Si support structure is generally defined at the level of the layer 2 near the silicon substrate 1.
This second solution cannot universally define the height of the etching. The position of the vertical bearing plane 8a (18a) of the support structure must be defined when assembling the module in accordance with the position of the woveguide 7 (17) relative to the bottom surface of the second optical component 6 (16). The height of the waveguide 7 (17) in the second optical component depends on the nature of that component and it would therefore seem difficult to control accurately the vertical alignment, along the z axis, of the waveguides 3, 7 of the two optical components.
Thus, unless penalizing constraints are introduced as to the transverse dimensions of the optical components, the above prior art solution cannot be used to align a plurality of optical components on a common structure; this would involve a plurality of successive etching operations in the support structure to define contact surfaces at different heights according to the position of the waveguide of each component.
Also, the smaller the waveguides, the more difficult it is to control alignment. Consequently, the assembly methods of the prior art cannot achieve accurate vertical alignment of the waveguides of different optical components and oblige support structure manufacturers to adapt their products according to the types of optical component that will be inserted in them.
The invention solves these alignment problems in that it proposes an assembly method in which the position of the vertical bearing point of each optical component is defined relative to the position of the optical axis of its waveguide.